The present invention relates to high-performance, high-temperature fiber-reinforced structural composites with carbon matrices.
Fiber-reinforced composites consist of two distinct components, fibers and matrix. Fibers, either continuous or in the form of short segments, are normally oriented in preferred directions in composites to utilize as much as possible the fiber""s great strength and stiffness properties. However, for low performance applications the fibers can be randomly placed to lower the cost of fabrication. Because fibers are heavily drawn and stretched during manufacture, they have properties superior to those of the same material in its undrawn and unstretched state; i.e., their bulk properties.
The matrix consists of material surrounding the fibers and has two purposes. The first is to fill the space between the fibers, which increases density and physically holds the reinforcing fibers in the preferred direction. The second is to transmit forces applied to the overall composite structure to individual fibers in such a manner as to distribute any applied forces, or loads, as nearly as possible to all fibers simultaneously. In this fashion, the high-performance fiber properties are retained by the composite since fibers bear more-or-less equal loads and hence do not break individually. This is accomplished with greatest success when all void spaces around fibers are filled in with matrix material. The void spaces are usually referred to as xe2x80x9cporosity.xe2x80x9d
For elevated temperature applications, high-temperature fiber-reinforced composites (HTFRC""s) are employed. These composites have excellent high-temperature strength retention, high strength-to-density ratio, and possess fracture toughness. In addition, the carbon-carbon (Cxe2x80x94C) composites have high specific modulus, good thermal conductivity and thermal shock resistance. High-performance HTFRC""s are used for structural applications in aerospace and rocket propulsion, such as, heat shields, leading edges and nozzles. To fabricate a high-performance HTFRC it is necessary to employ a high volume fraction (volume occupied by fibers/volume of composite) of the proper type and orientation of high performance reinforcement fibers that are held together in the composite by a high-density, high-quality matrix material.
Two categories of commercial processes have been developed to manufacture High-temperature Fiber-reinforced Composites (HTFRC) such as carbon matrix composites. These processes differ principally in the techniques used for the deposition of matrix materials around reinforcing fibers that have already been oriented and positioned into the locations they will occupy in finished products. One technique is vapor-phase in nature and is called xe2x80x9cinfiltration.xe2x80x9d The other is liquid-phase in nature, and is called xe2x80x9cimpregnation.xe2x80x9dBoth of these existing techniques share a common initial step. That is the formation of a xe2x80x9crigid-preformxe2x80x9d. This process can involve the holding of the fibers in the desired orientation and position in a mechanical frame and coating them with a suitable binder material, but usually involves the more simple steps of coating reinforcing fibers with a binder, which may be the same material as used to form the matrix, and then forming them into the desired shape by filament winding, hand lay-up, weaving, braiding, or some other means. This coated fiber preform is then heated to high-temperatures, with appropriate means taken to prevent loss of preform shape. The result of the heat treatment is the conversion of the binder to an inorganic cement. At this point any mechanical means of holding the fibers may be removed. The purpose of this cement, which can be produced from either a vapor or liquid hydrocarbon precursor, is to hold the reinforcing fibers in the shape desired for the final product. The ensemble of cemented fibers is called the rigidzed-preform, which is then subjected to subsequent processing. The task of heating the binder, or other materials used in HTFRC fabrication, to high temperatures to effect a change in chemical composition is usually referred to as xe2x80x9cpyrolysis.xe2x80x9d In most cases, this modification of the binder is from an organic to an inorganic substance. The cement formed by pyrolysis of the binder is very porous because of the relationship between pyrolysis efficiency and binder physical-property requirements. As mentioned previously, great care must be taken when handling high-performance fibers or the resulting damage will greatly diminish composite properties. This means that forces encountered by the reinforcing fibers during coating and positioning must be minimal. This can only occur if binder viscosity is low and care is taken in handling. Unfortunately, pyrolysis efficiency (the weight percent of binder remaining after pyrolysis) is usually found experimentally to increase only when binder viscosity is high. One solution to this dichotomy is the following current manufacturing methodology: keep rigid-preform performance potential high by utilizing low viscosity binders, and adjust for the resulting high initial porosity with subsequent processing. This subsequent processing to deposit material between the fibers in the preform is usually referred to as xe2x80x9cdensification,xe2x80x9d and is usually repeated many times.
As mentioned above, densification using existing technology takes one of two forms. The first is vapor-phase-based and involves placing the rigid-preform in an oven containing gases which decompose at high temperatures inside the preform to form carbon matrices. This process is referred to as chemical-vapor infiltration (CVI). The decomposition reaction is usually referred to as xe2x80x9ccrackingxe2x80x9d, since the splitting-apart of gas molecules is involved. However, it is also sometimes called pyrolysis, the same term used previously to describe similar thermal decomposition reactions occurring in solids and liquids. CVI has a number of problems associated with its use, the two most critical being pore closure at the surface leading to non-uniform densification, and poor matrix quality due to existence of multiple decomposition reaction-pathways leading simultaneously to multiple phases. Pore closure is detrimental because it denies access of infiltration gases to the preform interior. It occurs because cracking occurs more easily at solid surfaces. Thus, as gases attempt to enter rigid-preforms, decomposition takes place almost immediately on or near the hot exterior surfaces. This results in a density gradient through the sample, with a higher density matrix near the surface. This density gradient also limits the thickness of a high-performance part to less than 2xe2x80x3. The preferential deposition on or near the surface ultimately leads to the sealing off of the surface pore entrances in a relatively short period of time. Multiple phases are also harmful in most instances because they do not join together or consolidate well, making the matrix weak. These problems are both minimized to some extent by slowing down the CVI process. Also, partially-densified composites can be periodically removed from the CVI oven and have their surfaces machined away enough to reopen sealed pore entrances. This is, of course, very time consuming and adds expense. For carbon matrix composites, prior to or subsequent to machining, the partially-densified preform is heated to about 2400xc2x0 C. for long periods of time to convert the carbon matrix to a graphitic matrix. This process takes days to weeks and has associated high energy costs. The result of the steps described is processing times of many months, severe quality control problems, and associated high costs in both labor and energy.
The second densification process using existing commercial technology is liquid-phase-based. It involves impregnating rigid-preforms with liquid matrix-precursors and subsequently heating to high temperatures to initiate pyrolysis. It is in many ways similar to formation of rigid preform themselves, and suffers from the same drawbacks. Ease of impregnation and gentleness of handling are aided by lowered viscosity, but pyrolysis efficiency and matrix quality are enhanced by elevated viscosity. This is because the high viscosity matrix precursors produce a low porosity (high density) matrix which maximizes the physical integrity of the matrix thereby utilizing more and more of the reinforcing-fiber""s performance potential. Ideally, for maximum mechanical properties there should be zero matrix porosity.
However, because high viscosity liquid matrix-precursors do not easily enter and flow through pores in rigid-preforms, manufacturers of Cxe2x80x94C composites have enhanced entry and flow by immersing rigid-preforms in liquid matrix-precursor, heating the precursor to lower the viscosity, and applying pressure. This is necessary because these traditional liquid matrix precursors do not wet the perform as described below. This processing methodology does in fact help impregnation. However, it requires that heated pressure vessels be employed, which are very costly, and causes fiber breakage if pressures are changed too rapidly.
As with rigid-preform formation, the goal of attaining composite performance as high as possible presents a dilemma: use low-viscosity liquid matrix-precursors and obtain good impregnation, but poor pyrolysis efficiencies; or use high-viscosity liquid matrix precursors and obtain poor impregnation but high pyrolysis efficiencies. In either case, multiple liquid-densification steps will be needed because conversion of the matrix precursor normally results in a reduction of the matrix volume.
Because of the polymeric nature of these liquid matrix precursors, low viscosity can be achieved in the unadulterated state (i.e.; no solvents present) only by having the molecular weight low. This, in turn, reduces pyrolysis efficiency and affects matrix quality negatively, as explained earlier. Adding solvents to high-molecular-weight liquid matrix-precursors can certainly aid impregnation and result in improved matrix quality. But this approach actually increases the number of impregnation-plus-pyrolysis steps needed due to the high dilution ratios required to get acceptably low viscosities. Solvent rather than actual liquid matrix-precursor fills up most of the void space within preforms, and it must be removed prior to pyrolysis. This is a very time-consuming process. If solvent is not completely removed, any residual amount will turn into gas at temperatures far below those needed for pyrolysis. This results in either destruction of the preform (it literally explodes) or the expelling of high molecular-weight liquid matrix-precursor impregnated with the solvent.
In summary, manufacturers of carbon-carbon composites using a liquid phase precursor have therefore been faced with two choices. They can employ a low-viscosity liquid matrix-precursor and obtain good impregnation under pressure, filling most larger pores completely, but resulting in a small amount of low-quality matrix due to low molecular weight and poor pyrolysis efficiency. Alternatively they can utilize a high-viscosity liquid matrix-precursor forced in with higher pressure, which results in a small amount of better-quality matrix due to its higher molecular weight and good pyrolysis efficiency. However, on the down side, this high viscosity precursor does a poorer job of filling the porosity and will degrade composite properties resulting from the increased fiber breakage.
In addition, like CVI processes, current liquid phase processes seal off the surface pores and preferentially close off small pores, producing a billet with non-uniform density. In addition, these processes also require machining to open up the surface pore structure and graphitization to enhance the properties of carbon matrixes and open up internal pore structure. The result is again processing times of many months for high-matrix-density high-performance composites. It is the deficiencies of the long processing times, high costs, and non-uniform matrix density of such liquid-phase based processing which this invention addresses.
It should be noted that liquid binders used in rigid-preform formation and liquid matrix-precursors employed in densification can be, but usually are not, the same chemical substance. In Cxe2x80x94C composites, for example, the former is almost always phenolic in a solvent, while the latter is typically a refined petroleum or coal-tar pitch such as A240 or 15V. The fundamental chemical characteristic common to both liquid binders and liquid matrix-precursors used in all carbon matrix composites is that they are polymers. This fact explains why low-viscosity binders and precursors have low pyrolysis efficiencies and produce poor-quality matrices. In order to have low viscosity, polymers must possess a limited number of repeat units, otherwise entanglements between polymer chains occur during fluid flow limiting mass transport. During matrix formation by pyrolysis, the desired reaction is the loss of certain light constituents atoms, such as hydrogen, from polymer repeat units with no cleavage taking place between repeat units at all. However, in practice there is always unwanted but unavoidable side reactions in which there is the complete cleavage of individual repeat units off the ends of the polymer molecules thereby forming higher-molecular-weight gases. Since chain ends break off in cyclic fashion (i.e., one after another in rapid succession), pyrolysis yields are much lower in low-molecular-weight polymers than in high-molecular-weight polymers. High-molecular-weight polymers simply have far fewer chain ends to begin with, so there is much less end-breakage and associate gas evolution during pyrolysis. Gas evolution is detrimental because it pushes liquid matrix-precursor out of rigid-preforms before matrix formation by pyrolysis takes place and reduces the pyrolysis yield.
Moreover, high-molecular-weight polymers are better at aligning segments of their chain together than are low-molecular-weight polymers. The latter simply have too much mobility to stay aligned together for very long periods of time, especially at the high temperatures needed for pyrolysis to take place. They therefore tend to form poorly oriented or amorphous matrices. These typically have lower density and inferior physical properties. As stated before, the ideal liquid binder or liquid-matrix precursor should have high viscosity, at least from the viewpoint of pyrolysis efficiency and matrix quality. In light of the preceding observation, this would be due primarily to their having high molecular weights.
Two additional considerations, which are pertinent to understanding the negative aspects of existing liquid-densification techniques discussed above, are pressure impregnation, and wettability. Knowing that high viscosity liquid matrix-precursors do not easily enter and flow through pores in rigid-preforms, manufacturers of HTFRC""s have enhanced entry and flow by immersing rigid-preforms in a liquid matrix precursor and applying pressure. This does in fact help impregnation. However, it requires that heated pressure vessels be employed, which are very costly, and cause fiber breakage if pressures are changed too rapidly.
In contrast to industry thinking for the past 30 years, viscosity is not the dominant issue in impregnation. Rather, it is the lack of wettability that is principally responsible for the fact that pressure is needed to force precursors into the perform independent of their viscosity over certain ranges. That is, if a liquid wets a surface, it will tend to spread out on the surface and will automatically fill pores in the material. In contrast, if a liquid does not wet a surface, liquid droplets will sit on the surface and liquid will not spontaneously enter pores in the material. The pressure, P, needed to force a non-wetting fluid into a pore or capillary of radius, r, is given by the equation:
P=2xcex3 cos xcex8/r
where xcex3 is the surface tension of the fluid and xcex8 is the contact angle that the fluid makes with the surface. It can be seen from this equation that as the radius of the pore or void in the material decreases more pressure is required to force the non-wetting fluid into that pore. Thus, larger pores are filled first at lower pressure.
If pressure is employed, care must be taken not to break the fibers; consequently, pressure must be applied slowly. This will force the precursor into the larger pores in the perform preferentially and, depending upon the thermal treatment of the precursor while forcing the material in, smaller pores can be blocked and remain unfilled. In addition, as the precursor is converted to carbon, shrinkage occurs creating additional void space. In many cases, this void additional space is too small to be filled by the applied pressure and it therefore remains unfillable becoming closed porosity.
It is known from work in our laboratory that during fabrication of Cxe2x80x94C composites, for example, high-molecular-weight liquid matrix precursors do not wet fiber surfaces, whereas some low-viscosity low-molecular-weight liquid matrix-precursors at least partially wet fiber surfaces. In general, our work has shown that, for a particular series of pitches, the lower the molecular weight, the better liquid matrix precursors wet fiber surfaces, as measured by contact angle. However, it should be stated that low molecular weight and low viscosity do not guarantee wetting of the matrix material on a particular surface. Whenever liquid matrix-precursors possess good wetting properties impregnation is greatly aided because the precursors readily soak into rigid-preforms in much the same fashion as water soaks into cotton fabric.
In contrast, if the matrix precursor does not wet the fibers as in current processing technology, pressure will be needed to force the matrix precursor into the preform. This will produce an uneven distribution of matrix precursor in the preform. The result being a higher density near the surface than the center of the preform as well as the closure of surface pores. To try to compensate for these two shortcomings, i.e., inability to density the billet uniformly and the associated surface pore closures, manufacturers of carbon matrix composites, for example, machine the outside of the billet to open up the pore structure and then re-impregnate, carbonize, graphitize and machine up to eighteen times. This is an extremely time consuming, labor intensive and costly process that can take up to eighteen months for a large billet.
The commercial manufacture of carbon-carbon composites has taken place for more than 30 years and is a rather mature field. Both chemical vapor infiltration (CVI) and liquid phase impregnation techniques (or a combination of the two) have been used to place the carbon matrix in the rigidized preform.
During this time the goal has remained the same: to be able to produce a thick ( greater than 2xe2x80x3) billet with uniform density at low cost. This objective not been obtained to date commercially due principally to the matrix precursor employed. Conventional gas phase chemical vapor infiltration processes using hydrocarbon precursors are not able to uniformly densify a large-thick billet of complex shape because of the preferential deposition on the outer portion of the billet and the inability to control concentration and temperature gradients in the gas phase. In addition, this family of processes is very expensive due to the expensive equipment and the long processing times required. Attempts to solve the surface deposition problem have involved using a pressure gradient alone or in conjunction with a temperature gradient (hotter on side opposite gas entry) through the part to be densified In addition, a temperature gradient through the part utilizing a heater in the center in conjunction with surface cooling involving a liquids latent heat of vaporization has been employed. All three approaches have met with some success. However, these techniques are still very costly and limited to relatively small and thin parts with little shape complexity. However, it should be mentioned that the combination of forced flow and a reversed temperature gradient has increased the thickness that can be densified with reasonable uniformity to nearly two inches. Liquid-phase matrix precursors have included neat organic resins, particulate loaded resins, as well as all types of petroleum and coal tar pitch materials. The patent literature contains many processes that utilize various organic resins, coal tar pitches, petroleum pitch solvent-refined pitches, particulate loaded resins, and super-critically-refined pitches.
The ability to produce low cost composites with uniform density using liquid-phase carbon precursors has been hindered by the conflicting demands of high char yield and low viscosity. Processes using various organic resins as well as coal tar and petroleum pitch suffer from the fact that these materials have low char yield and high viscosity unless solvated. In addition, these materials do not meet the critical criteria of wetting the fiber preform surface. Processes that involve the use of solvent-refined pitches, super-critically-refined pitches, and mesophase liquid-crystal polymer have increased the char yield but have not addressed the wettability issue. Thus, they still require many costly processing cycles to produce a composite that is not uniform in density. The use of carbon particulate loaded resins again increases the char yield. However, these processes suffer from the same problems as non-loaded resins and in addition are not able to density a thick composite. In fact, they actually produce a lower quality composite because the particles block the pore structure on the first cycle and limit subsequent densification.
Currently, the matrix precursor material of choice is a mesophase liquid crystal polymer made from petroleum pitch using various proprietary temperature-pressure cycles. The use of mesophase pitch brings up one last factor which is pertinent to the understanding of the shortcomings of some existing liquid densification techniques for carbon matrix composites. This is the polymerization pathway used to form the matrix precursor. Since the high-char-yield mesophase pitch, for example, is too viscous to use for impregnation and does not wet the preform surface, the preform is impregnated with low-viscosity, low-char-yield isotropic pitch, which is able to wet the preform surface. This pitch is then converted to mesophase pitch inside the preform using various temperature-pressure cycles. The problem with this technique is that it involves a two-phase addition polymerization process since the mesophase is not miscible in the isotropic pitch from which it is made. Thus, when the size of the liquid crystal mesophase spheres formed in the isotropic pitch within the preform exceeds the size of the space they occupy, they are expelled and replaced with the isotropic pitch material which forms a lower quality matrix.
In contrast to current densitication processes, using isotropic petroleum pitch, with the present invention there is only a single phase present within the perform because mesophase is produced directly from the monomer without any phase change. That is, after the low-molecular-weight wetting monomer coats the preform and fills the voids, it is polymerized into a miscible high-molecular-weight polymer. Although, this high-molecular-weight condensation polymer does not wet the surface, it is not expelled if there is a single phase because it is highly viscous.
The historical evolution of carbon matrix precursors from as-received pitch materials to mesophase pitch, has developed to try to improve the quality of the matrix microstructure and to attempt to solve the problem of uniform through-the-thickness density in the finished billet. Over the years the matrix microstructure has been improved but no process to date has been able to uniformly densify a thick preform. This is because the universal criteria for efficient impregnation of the preform has been viscosity. No one has used the more important criteria of wettability in selecting the best matrix precursor. Thus, all the processes in the patent literature rely on temperature to lower the viscosity, and pressure or a combination of vacuum and pressure to force the non-wetting matrix precursor to into the rigidized preform. This is a very inefficient process that preferentially fills larger pores, seals off smaller pores, and densities the exterior of the preform at the expense of the interior. As a result of using a non-wetting matrix precursor or matrix material, many impregnation-carbonization-graphitization-machining-cycles are required to reach a density of 1.9 g/cc. This equals many months of processing at a cost that keeps the market for carbon-carbon small. To add insult to injury, even though the final product is very costly, it does not have uniform density through the billet. It is clear that there is a great need for a low-cost impregnation technology that produces a billet with uniform density and good mechanical properties.
Currently, cost is the main factor that limits the application of carbon-carbon in many areas. Approaches to lower cost have included using low cost fiber, low cost matrix material, adding particulate fillers to the matrix, using random orientation of fibers, as well as molding and hot-pressing techniques. However, in an attempt to lower cost, performance has been degraded to such an extent that it precludes the use of carbon-carbon composites made by these processes in many high-performance structural applications. What is needed is a means to significantly lower the cost of carbon-carbon composites while maintaining or enhancing the composite properties and performance. Since the main cost of carbon-carbon composite fabrication is associated with the densification process, there is a need for a low cost liquid phase densification technique.
Although this invention only addresses liquid phase impregnation, it should be stated that all gas phase infiltration techniques known to date suffer from the same drawbacks as just stated for liquid phase infiltration. That is, they are very time consuming, very costly and are not capable of producing a thick billet with uniform density through the thickness. In fact, CVI processes are even more inefficient than liquid phase processes in densifying the center of a billet. As a result, CVI processing is not normally used to attempt to densify thick preforms. Therefore, there is a more general need. That is, for a densification process that will produce carbon composites at low cost and with uniform density and excellent mechanical properties.
We have found that carbon-carbon composites can be fabricated in a manner that is not only economical, but overcomes most, if not all, the drawbacks of the techniques discussed hereinbefore. We have found that high-performance HTFRC""s can be fabricated by impregnating composite preforms with liquids, i.e., monomers, which wet the fibers and subsequently undergo polymerization and carbonization reactions in the perform to create carbon matrices. These preforms include, but are not limited to: externally mechanically constrained performs; performs rigidized with a binder; and performs internally constrained by weaving, braiding, flocking, or entanglement.
The use of a monomer that wets the preform and the partially-densified preform surface is a distinctive point of this invention. Although lower viscosity increases the rate of impregnation into the perform, it is not the key factor that it is in other processes. In the densification process of this invention viscosity does not determine whether or not the impregnant will go into the perform but only the rate at which it enters. Thus, the required viscosity of the wetting monomer is based on economic and schedule constraints rather than technical requirements. Once the monomer has been drawn inside the preform, another feature of this invention is the initiation of polymerization of the monomer molecules, which we refer to as xe2x80x9cIn Situ Densificationxe2x80x9d. After polymerization takes place, the resulting liquid matrix-precursor will acquire the high molecular weight needed to produce a superior matrix (upon pyrolysis) with high efficiency. The molecular weight of the polymer can also be controlled in this invention in order to tailor composite properties. Thus, this invention combines the low viscosity of the monomer and the high char yield of the polymer along with the wettability of the monomer to produce a high quality uniform matrix without the need for costly pressure vessels and long processing times with associated high costs.
In the wetting or nonwetting of solids by liquids, the criterion employed is the contact angle between the solid and the liquid, as measured through the liquid. A liquid is said to wet a solid if the contact angle is between 0 and 90xc2x0, and not to wet the solid if the contact angle lies between 90xc2x0 and 180xc2x0. A mundane example of the wetting/nonwetting phenomenon may be seen on a dirty/clean automobile during a rain shower: on a clean automobile the rain droplets bead on the surface, an example of nonwetting, while on a dirty automobile the rain droplets spread out, an example of wetting.
Accordingly, it is an object of the present invention to provide a high-temperature fiber-reinforced composite with essentially uniform density and an insignificant amount of closed porosity.
Another object of the present invention is to provide a carbon-carbon composite with high density, high strength, high modulus and high thermal conductivity without graphitization.
Yet another object of the present invention to provide a process for making a high-temperature fiber-reinforced composite with essentially uniform density and an insignificant amount of closed porosity.
A further object of the present invention is to provide a process for making a carbon-carbon composite with high density, high strength, high modulus and high thermal conductivity without graphitization.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the present invention there is provided a process for manufacturing a high-temperature fiber-reinforced carbon composite material of essentially uniform density, which comprises the steps of:
(a) selecting a fiber/matrix material combination;
(b) providing a fiber preform of desired shape and fiber placement;
(c) selecting at least one low-viscosity pre-carbon monomer material that wets the surfaces of the fiber preform;
(d) impregnating the fiber preform with the monomer;
(e) polymerizing the monomer material in-situ in a single phase process into a pre-carbon polymer of desired molecular weight;
(f) pyrolyzing the pre-carbon polymer to form a carbon matrix material; and
(g) repeating steps (d)-(f) to further densify the preform.
The resulting composites consist of a high volume fraction of high-performance carbon reinforcement fibers that can have any number of dimensions (1-D, 2-D, 3-D, . . . n-D) of orientation that it is possible to construct by felting, weaving, braiding, lay-up, etc. In general, what constitutes a high-fiber-volume-fraction depends on the number dimensions of fiber reinforcement, but an average value for a high performance composite is 45%. These composite materials are unique in that they have a uniform density through the composite and the smallest voids are completely filled preferentially by capillary action and remain filled during processing. This means that in one densification cycle the voids within the fiber bundles or tows are completely filled with matrix tightly bonded to the fibers allowing highly efficient transfer of load between the fibers. The result of this complete fiber bundle filling is that after one densification cycle, even though the matrix pockets have not been completely filled, these composites can have the properties of multi-cycle heat-treated (graphitized) composites of similar density. Thus, it is not required (although this is optional) to go through the costly and time-consuming graphitization process in order to achieve desired properties.
As used herein, the term xe2x80x9chigh-densityxe2x80x9d means at least 80% of theoretical density.
It is also within the scope of this invention to use more than one monomer to densify the preform. These monomers can be introduced into the preform together during the same cycle in order to ultimately form a hybrid matrix material or they can be introduced during different cycles to form a layered structure for additional fracture toughness. With this technique one can also tailor the molecular weight and make it whatever desired. The minimum molecular weight is that required to form mesophase liquid crystals which are three dimensional molecules composed of non-linear planar molecules that are not cross-linked.